Within integrated circuits, a need frequently arises for a stable temperature independent reference signal substantially free of thermal drift, commonly generated using a band gap circuit. FIG. 4 depicts one possible implementation of a band gap circuit, circuit 400.
Fabrication of band gap circuit 400 in a manner producing consistent output, with only negligible variation between different instances, is difficult. From one lot to another, or across different fabrication processes or sites, the temperature dependence of the output voltage for different instances of the circuit may vary. For this reason, adjustment of the band gap circuit's output voltage temperature dependence may be necessary in specific instances.
The temperature dependence of a standard band gap circuit is typically adjusted by either injecting currents into the emitter of one of bipolar junction transistors Q1 or Q2, or by adjusting the resistance Rp. However, because the output voltage is regulated in a standard band gap circuit, the rail voltages (the upper and lower power supply voltages) generally must be several hundred millivolts (mV) greater than the actual output voltage, which can significantly impact the circuit's power consumption and usefulness.
In low power/low voltage (e.g., battery powered) applications, the output voltage of the band gap circuit is typically left unregulated, allowing the band gap circuit to have an output voltage lower than the band gap of silicon and to operate on a rail voltage that is lower than the band gap of silicon.
Adjusting the output voltage temperature dependence of a low voltage band gap circuit by adjusting resistance Rp is not advantageous since the voltages on either side of that resistor may not be sufficient to allow a switch to turn on, which is especially true as the rail voltage approaches its minimum. Moreover, while modifying the value of the resistance Rp by laser trimming is possible, such processing adds significant expense and is therefore undesirable. Injecting a current into transistor Q1 or Q2 in a low power band gap circuit is not optimal since the temperature coefficient of the injected current would not likely match the temperature coefficient of the current normally passing through those transistors, which will effect the temperature dependence of the output voltage.
There is, therefore, a need in the art for cost-effective adjustment of the temperature dependence for low voltage band gap circuits.